Epitaxially grown silicon-based single-atom catalyst for visible-light-driven syngas production

Improving the dispersion of active sites simultaneous with the efficient harvest of photons is a key priority for photocatalysis. Crystalline silicon is abundant on Earth and has a suitable bandgap. However, silicon-based photocatalysts combined with metal elements has proved challenging due to silicon’s rigid crystal structure and high formation energy. Here we report a solid-state chemistry that produces crystalline silicon with well-dispersed Co atoms. Isolated Co sites in silicon are obtained through the in-situ formation of CoSi2 intermediate nanodomains that function as seeds, leading to the production of Co-incorporating silicon nanocrystals at the CoSi2/Si epitaxial interface. As a result, cobalt-on-silicon single-atom catalysts achieve an external quantum efficiency of 10% for CO2-to-syngas conversion, with CO and H2 yields of 4.7 mol g(Co)−1 and 4.4 mol g(Co)−1, respectively. Moreover, the H2/CO ratio is tunable between 0.8 and 2. This photocatalyst also achieves a corresponding turnover number of 2 × 104 for visible-light-driven CO2 reduction over 6 h, which is over ten times higher than previously reported single-atom photocatalysts.

Here, we discuss challenges in quantification of Co atoms using electron microscopy techniques in our study.
One can apply the quantitative analysis technique "atom counting" to quantitatively analyze both positions and numbers of atoms inside a nanomaterial 14 . Moreover, it is possible to capture an EELS signal of single atoms. However, it should be noted that colloidal single-atom catalysts are typically quite electron-beam sensitive and thus single-atoms can delocalize upon electron beam illumination. To reduce the electron beam damage, we have used graphene TEM grids (to reduce the background and potential electron beam damage) and relatively low beam current during our electron microscopy measurement. Still, possible structural changes induced by the electron beam may influence the reliability of the measurements when attempting to capture EELS signal or quantitative atom counting for single Co atom.
Another challenge that hampers the ability of locating single Co atoms is unavoidable electronbeam induced carbon contamination. As a mitigation, we applied activated charcoal based preclean treatment for the single-atom catalysts prior to TEM measurements to absorb isolated ligands and potential organic residuals 15 . Still, a rapid growth of carbon layer under electron beam leads to a continuously increasing background signal during EELS measurement. Moreover, the carbon contamination induced inhomogeneous background intensity, which makes it more challenging to achieve a reliable quantification of positions and numbers of single Co atom inside the catalysts.
Finally, the amount of Co atom is quite low, hence it is quite challenging to extract background (e.g., carbon) to localize the EELS signal of those single atoms and therefore correlating with atomic positions based on EELS mapping.
We foresee that extra efforts on quantitative imaging analysis technique are needed to tackle challenges in quantification of atomic structure of beam-sensitive materials in the future.

Supplementary Note 2 | Functions of crystalline Si component on Co@Si SACs for CO2RR.
To better elucidate the catalytic behaviour of the active Co sites, we removed the SiO2 matrix and CoOx to liberate Co@Si NCs by HF etching 16 . The colloidal Co@Si NCs were subsequently used for CO2RR under the same reaction condition for 9 h (Supplementary Fig. 17). The yield of CO (200 μmol g -1 ·h -1 ) is 18 times higher than the reaction with Co-free Si NCs but significantly lower than the Co@Si SAC sustained by porous SiO2 matrix, indicating that the silicon-based Co@Si nanostructures play a crucial role in CO2-to-CO conversion. More importantly, efficient CO2RR performance can be achieved without applying additional photosensitizer. This may be attributed to the intense light absorption capability of c-Si, which enhances the photoelectron generation and the subsequent charge carrier separation on Co@Si SACs ( Supplementary Fig. 16).

Supplementary Note 3 | Influences of the proton donor and the sacrificial reagent on the H2/CO ratio in the syngas product.
We carried out control photocatalytic experiments with a varied amount of water and sacrificial reagent to investigate their impact on the product types and yields. Results show that the number of protons has a dominant influence on photocatalytic performance ( Supplementary   Fig. 19). The H2/CO ratio can be tuned between 1 -2 by varying the usage of H2O ( Supplementary   Fig. 19a), indicating that water plays a key role generation of protons. We also found that the Co concentration of Co@Si SACs and the amount of sacrificial agent have a minor influence on the H2/CO ratio ( Fig. 4b and Supplementary Fig. 19b). We therefore conclude that the number of protons is more vital to the H2/CO ratio in syngas production.